The present invention concerns a vapor phase process for the deposition of a protective coating on a metallic piece, the apparatus for carrying out the process, and the pieces obtained by said process.
A process for the protection of metal pieces, in particular refractory alloy pieces, at elevated temperatures, with a high resistance to deformation by mechanically and thermally generated stresses, by coating the surfaces of said pieces with a suitable metal, has already been described by French Pat. No. 1,433,497. This process consists of contacting said metal pieces, located in a suitably heated enclosure, with a halide of the metal to be deposited; said halide, gaseous at the temperature of said enclosure, being formed by the reaction at an elevated temperature of a chemical halide compound with a derivative (metal or alloy or chemical product) of the metal to be deposited.
Modes of application of this general method have been described in particular in French Pat. Nos. 2,134,220 and 2,181,512 for titanium carbide, nitride or carbonitride coatings.
In the basic French Pat. No. 1,433,497 and in the modes of application described in the U.S. Pat. Nos. 2,134,220 and 2,181,512, the "donor", i.e., the metal derivative to be deposited, are used in the form of particles, i.e., of more or less coarse grains, which are placed advantageously in baskets located in the heated enclosure wherein the coating is effected.
It has now been discovered that the form wherein the donor is used may have a certain influence not merely on the facilities to effect the process, as described, but also on the results that may be obtained.